The overall aim of this proposal is to explore methods for blocking gene expression in the mouse in vivo. Initially, we will use as a test system the production of the protease tissue plasminogen activator (t-PA). This enzyme is produced in high amounts in maturing mouse oocytes, early embryos, and various somatic tissues. However, its physiological role is not defined. We will use two strategies for interrupting t-PA expression: 1. We will produce transgenic mice that express antisense RNAs specifically in the oocyte. Included will be approaches that exploit recent information about the control of transcription in an attempt to design new strategies for the use of antisense RNA. 2. We will disrupt one or both copies of the t-PA gene in mice via homologous recombination. These studies should give new information about the role(s) of t-PA in oogenesis, early embryogenesis, and mammalian physiology. They may also identify better methods for gene disruption in mammals that can be extended to other gene products.